The development by the National Aeronautics and Space Administration (NASA) of Space Station Freedom has fostered a new impetus toward the development of a completely automated rendezvous and docking system. Many future missions into space will involve rendezvous and docking scenarios that are virtually impossible, risky or uneconomical if ground piloted control of the docking procedure is used because of the long time delays involved. Even operations taking place in Earth orbit are subject to time delays on the order of several seconds.
Initially, the rendezvous or docking of two spacecraft was primarily a manual operation. Such maneuvers were executed by highly trained astronauts, visually acquiring and tracking a target vehicle, while manipulating control mechanisms to fly the chase vehicle during the last roughly 1000 feet of the vehicle trajectory to a desired docking point. Although radar and inertial measurement data were often available to the astronaut so as to provide more accurate information about the spacecraft's position and attitude, the burden was on the astronaut to visually estimate the attitude of the target vehicle relative to his own chase vehicle. Some vehicles included three-axis (roll, pitch and yaw) autopilots to assist in maintaining vehicle attitude. However, these autopilots are incapable of generating the control signals necessary for translation (x, y and z axis) maneuvers.
It will be appreciated by one skilled in the art that using a human astronaut for docking a space vehicle involves a number of disadvantages in addition to those discussed above. For example, there are inherent limitations on the accuracy of an astronaut's estimates with respect to geometrical relations (i.e., angles and distances). Likewise, humans lack the ability to exactly repeat the same control inputs. Therefore, such considerations must be factored into vehicle design as well as mission planning in order to ensure that the required margin of safety is provided resulting necessarily in increased costs and decreased operational flexibility.
To illustrate, docking mechanisms must be overdesigned in order to withstand impacts at high velocities that may occur with astronaut error. In addition, costly time-consuming "pilot-in-the-loop" simulations must be performed to verify the suitability of the information provided to the pilot in both normal and contingency situations.
In response to the inherent limitations and deficiencies of manual docking operations, unmanned docking systems were created. In a limited number of situations, these systems can be successfully accomplished by teleoperations or remote control. Such techniques rely on downlink telemetry of TV images which are monitored by ground control. Guidance commands are then transmitted back to the vehicle on the telemetry uplink. However, systems using communication uplinks usually require a wide bandwidth because both high rate telemetry and video displays are normally needed by ground control which, in turn, tend to be costly and invariably encounter interference problems. Furthermore, the end-to-end time delay introduced by the communications link will degrade the performance of the ground control in docking the chase vehicle, thereby increasing the chances of failure. Therefore, the large number of future docking missions and the vastness of space eliminate the use of teleoperations in many scenarios.
Similarly, although Radio Frequency (RF) technology has long been developed for acquiring and tracking targets for various purposes, the magnitude of RF wavelengths preclude operation at very close ranges and cannot be adapted for the precision measurement requirement required for automated docking. Moreover, RF systems also tend to be plagued with interference problems.
In response to the above deficiencies, various attempts have been made to improve on the unmanned docking of two spacecraft. Interestingly, U.S. Pat. No. 5,109,345 discloses a non-synchronized passive docking system comprising a passive reflector target having three reflective areas thereon affixed to a target vehicle and a tracking sensor attached to a chase vehicle.
The system further comprises a laser diode array mounted on the chase vehicle for illuminating the reflective areas on the passive reflector target which causes light to be reflected therefrom. Substantial eye-safety problems are present if humans occupy the target vehicle. The tracking sensor detects the reflected light and produces electrical output signals in accordance with reflected images thereof.
As noted above with the teleoperated docking systems, this passive reflective docking system possesses many drawbacks. A major disadvantage of this system is that it requires a great expenditure of optical power for an adequate return of illumination from the target. This is particularly true for large targets when there is especially large distances between the chase vehicle and the target. Thus, passive reflective docking systems must illuminate the target with high-powered lasers. When such a system is used with a manned target vehicle (such as Space Station Freedom), human safety is compromised in that the occupants will be at extreme risk of exposure to laser light.
Moreover, the passive reflector areas on the target are frequently subject to bombardment by micrometeorites. There is a very good probability of target damage to a point where their reflectiveness is inadequate. Unfortunately, this damage may not be discovered until a docking maneuver is attempted, which then may prove to be too late.
In addition, the passive reflective system must also expend a great amount of internal processing power to omit data received from a "non-reflecting" target (i.e., noise). In essence, since the system is non-synchronized, it must "subtract" all of the pictures of a "non-lit" target from a picture with the target lit per cycle in order to generate adequate docking commands, otherwise there would be an unacceptably small signal-to-noise ratio. Clearly, a non-synchronized system having a camera operating at 30Hz or more will require substantial internal processing power.
Preliminary work in this field relating to autonomous docking systems includes that described in Tietz, J.C. and Kelly, J.H.: Development of an Autonomous Video Rendezvous and Docking System, Martin Marietta Corporation, Contract No. NAS8-34679, Phase One, (June 1982); Dabney, Richard W.: Automatic Rendezvous and Docking, A Parametric Study, NASA Technical Paper No. 2314, (May 1984); Tietz, J.C. and Richardson, TOE.: Development of an Autonomous Video Rendezvous and Docking System, Martin Marietta Corporation, Contract No. NAS8-34679, Phase Two, (June 1983); and Tietz, J.C.: Development of an Autonomous Video Rendezvous and Docking System, Martin Marietta Corporation, Contract No. NAS8-34679, (Jan. 1984).
While the above-mentioned systems are suited for their intended purpose, none of them disclose a Global Positioning System Synchronized Active Light Autonomous Docking System (GPSSALADS) wherein a target vehicle incorporates an active light target having active illuminators which flash intermittently and which are synchronized with a visual tracking sensor and image-processing circuitry on a chase vehicle by time signals received from a standard Global Positioning System receiver located on both the chase and target vehicles.
Inasmuch as the art is relatively crowded with respect to these various types of passive reflective docking systems, it can be appreciated that there is a continuing need for and interest in improvements to such systems, and in this respect the present invention addresses this need and interest.